1. Field of the Invention
This invention relates to a predistortion circuit for compensating the non-linearity of an amplifier.
2. Description of the Prior Art
Predistortion circuits for compensating the non-liner characteristic of a high frequency power amplifier are known. If a plurality of carrier signals having different frequencies are inputted to a high frequency power amplifier having a non-linear characteristic, such as the last stage of a radio apparatus or a video apparatus, intermodulation occurs. For example, if it is assumed that the frequency of the first carrier signal inputted to a power amplifier is f1, the frequency of the second carrier is f2, wherein f2&gt;f1, spurious outputs such as third intermodulation distortion components of (2f2-f1) and (2f1-f2) and fifth intermodulation distortion of (3f2-2f1) and (3f1-2f2) are developed. As mentioned above, if there is intermodulation, various spurious outputs are developed. This may interfere the neighbor channels. Then, the minimum suppression amount (IM3) of the third intermodulation distortion component of the carrier signal and that of the fifth intermodulation distortion component of the carrier signal are specified. Thus, in the amplification characteristic, a suppression amount higher than the specified suppression amount is required. However, because the power of the fifth intermodulation distortion component is smaller than the third one, it is actually enough to suppress the power of the third intermodulation distortion component under the predetermined value.
Recently, in the mobile communication systems, a multi-value modulation such as 16QAM having much amplitude variation or a multi-carrier system are proposed to provide the high speed data transmission in addition to voice signals. Thus, the power amplifier for a base station is required to have a high linearity in addition to the high output and high efficiency.
On the other hand, a saturation type of amplifier having a non-linear characteristic is used for the power amplifier in consideration of the power efficiency. Then, the predistortion circuit having the non-linear characteristic that has an inverse relation with the non-linearity of the power amplifier having a non-linear characteristic is provided before the power amplifier. Thus, a high linearity is obtained by suppressing the third intermodulation distortion component due to the non-linier characteristic of the power amplifier.